For an object captured via an image pickup optical system, light emitted from one point cannot be converged to another point and has a minute spread due to an influence of a diffraction, an aberration, or the like that occurs in the image pickup optical system. Such a minutely-spread distribution is referred to as a PSF (point spread function). Due to the influence of the image pickup optical system, the captured image is formed with the PSF convolved with the image, and accordingly, the image is blurred and its resolution is deteriorated.
When an object image (optical image) is acquired as a digital image, the optical image is discretized with pixel pitch intervals by an image pickup element. When a frequency response of the optical image exceeds Nyquist frequency of the image pickup element, aliasing occurs. Due to the generation of the aliasing, a moire (periodic pattern) occurs in the digital image. In the image pickup apparatus such as a digital camera, typically, the generation of the moire is not preferable, and in particular a color moire is also called a false color. As a method of reducing the generation of the moire, a method of inserting an optical low-pass filter to attenuate the frequency response near the Nyquist frequency of the image pickup element is known.
On the other hand, PTL 1 discloses an image pickup apparatus that captures a plurality of images while driving optical lenses (image pickup optical system) minutely to acquire an image in which any moire is not generated. PTL 2 discloses an image processing method that stores a frequency at which the moire occurs based on a relationship between a dot density at the time of printing and a surface density of an image itself and that specifies a region of the image where the moire occurs based on frequency characteristics of the image. NPL1 discloses linearity between color components of a locus region in an image, and it discloses a method of reducing a color noise by improving the linearity between the color components when the color noise occurs. NPL2 discloses a method of reducing a phenomenon where the linearity between the color components is collapsed like the color noise.